This invention relates to a detecting circuit for a power source voltage which is composed of a reference voltage generating circuit, a divider circuit for the power source voltage and a voltage comparing circuit.
In the conventional type, a small type mercury battery is employed as an energy source for powering small electronic equipment such as electronic timepieces and calculators. This type of mercury battery must be able to generate about 1.6 Volts and to constantly maintain a constant voltage for a long time, however, with such batteries the voltage suddenly falls after elapse of certain time. Accordingly, it is necessary to detect the battery voltage and to change the battery before its voltage suddenly drops causing instability of the operation of the electronic equipment. In the conventional detecting circuit for a power source voltage, many circuits have been developed such as shown in FIGS. 1 to 3. These prior art circuits are composed of a combination of MOS-transistor and resistor whereby they can be easily fabricated using known integrated circuit techniques. However, irregularities in the electro-conductive coefficient or voltage threshold of the MOS-transistor and in the resistance value of the diffusion resistance are caused during the fabrication process of the IC. As means for eliminating such irregularities, it is necessary to utilize an outer electronic part for adjusting the battery life detecting voltage "Ej". Actually, one resistor in the circuit is employed as the outer electronic part thereof.
The use of outer electronic parts is very disadvantageous from the standpoint of designing very small electronic equipment such as an electronic wrist watch and calculator, particularly since in small electronic equipment it is desirable that all of the electronic parts thereof are completely made by IC-construction. Further, the working process for adjusting the resistance and aligning the battery life detecting voltage "Ej" is very difficult and result in a higher cost.